mpls: Prevent use of implicit NULL label as outgoing label

The reserved implicit-NULL label isn't allowed to appear in the label
stack for packets, so make it an error for the control plane to
specify it as an outgoing label.
Suggested-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Robert Shearman <rshearma@brocade.com>
Reviewed-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mpls/af_mpls.c

@@ -646,6 +646,15 @@ int nla_get_labels(const struct nlattr *nla,
 		if ((dec.bos != bos) || dec.ttl || dec.tc)
 			return -EINVAL;
 
+		switch (dec.label) {
+		case LABEL_IMPLICIT_NULL:
+			/* RFC3032: This is a label that an LSR may
+			 * assign and distribute, but which never
+			 * actually appears in the encapsulation.
+			 */
+			return -EINVAL;
+		}
+
 		label[i] = dec.label;
 	}
 	*labels = nla_labels;